The subject matter disclosed herein relates to X-ray tubes used in medical imaging and, in particular, to the thermal control of X-ray tubes.
In non-invasive imaging systems, X-ray tubes are used in fluoroscopy, projection X-ray, tomosynthesis, and computer tomography (CT) systems as a source of X-ray radiation. Typically, the X-ray tube includes a cathode and a target. A thermionic filament within the cathode emits a stream of electrons towards the target in response to heat resulting from an applied electrical current, with the electrons eventually impacting the target. Once the target is bombarded with the stream of electrons, it produces X-ray radiation and heat.
The X-ray radiation traverses a subject of interest, such as a human patient, and a portion of the radiation impacts a detector or photographic plate where the image data is collected. Generally, tissues that differentially absorb or attenuate the flow of X-ray photons through the subject of interest produce contrast in a resulting image. In some X-ray systems, the photographic plate is then developed to produce an image which may be used by a radiologist or attending physician for diagnostic purposes. In digital X-ray systems, a digital detector produces signals representative of the received X-ray radiation that impacts discrete pixel regions of a detector surface. The signals may then be processed to generate an image that may be displayed for review. In CT systems, a detector array, including a series of detector elements, produces similar signals through various positions as a gantry is displaced around a patient.
The X-ray tube has a useful life over a large number of examination sequences, and must generally be available for examination sequences upon demand in a medical care facility, as examination sequences may or may not be scheduled, for example due to emergency situations. When the X-ray tube is not in use, the X-ray tube may cool between imaging sequences, as no electrons are being emitted by the thermionic element (i.e., substantially no heat is being generated). This cooling may result in the target material dropping below its ductile to brittle transition temperature, which can result in fracture of the target or reduced operating life. Existing techniques to warm X-ray tubes are often unreliable and inefficient, as typical thermal transition processes may take up to one hour and can over or undershoot a desired target temperature, resulting in instability of the target material. In such cases, image sequences may be delayed or, in cases where imaging sequences are performed before the target is properly warmed, the target may rupture. Accordingly, a need exists for improved thermal control in X-ray tubes.